Update RxJS Observable URL to newer version + Fix some spelling mistakes

[michaelg100]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/docs.nestjs.com/blob/master/CONTRIBUTING.md

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Docs
• Other... Please describe:

What is the current behavior?

Currently the observable streams url point to: http://reactivex.io/rxjs/class/es6/Observable.js~Observable.html. However if you're on that site and click home it will bring you to their newer doc site.

Also a few things are currently misspelled.

Issue Number: N/A

What is the new behavior?

Updating the link to point to the latest observable docs here: https://rxjs-dev.firebaseapp.com/guide/observable.

Also, fixing some misspellings.

Does this PR introduce a breaking change?

• Yes
• No

Other information

N/A

[kamilmysliwiec]

lgtm